The relation between the concentration of placental specific proteins in retroplacental and peripheral blood.

Concentrations of four placental proteins: human placental lactogen (hPL), placental protein 5 (PP5), pregnancy specific beta 1 glycoprotein (SP1) and human chorionic gonadotrophin (hCG), and a normal serum component, alpha 2 macroglobulin, were measured in the peripheral circulation and in blood obtained from the retroplacental space in 20 women at term delivery. Levels of hPL and PP5 were higher in the retroplacental blood than in the peripheral circulation in all patients. By contrast, levels of SP1 and hCG were consistently lower in retroplacental blood than in the peripheral circulation. Similarly, levels of alpha 2 macroglobulin were lower in the retroplacental blood. It is suggested that this 'reverse' gradient is a technical arterfact. These findings are discussed in relation to synthesis of placental proteins in a site distal to the retroplacental space, and the introduction of a technical artefact in the collection of samples.